Larger output size or higher resolution scan?

[shannaford]

Hi all, hopefullly a fairly easy question on scanning.
I want to get some 24" X 30" prints done from some B&W 4x5 negs. Printing on a Lambda, the lab wants files submitted at desired size (eg 24x30), 300dpi (which sounds fair enough to me).
Not having a lot of experience, having only used Vuescan and Silverfast a couple of times, what is the best way for me to get to the desired file size?
Should I be looking to scan the neg at 300dpi with an output size of 24" x 30", or should I be setting output at actual size of the neg, but scanning at higher resolution? Does it matter which way I go?
Thanks,
Scott

[percepts]

The way to work it out is to work backwards from the print size using the longest print dimension.
So the longest size is 30 inches. At 300 pixels per inch output, that is 30x300 = 9000 pixels. So your scanned neg needs to give 9000 pixels width. If your neg image area is 5 inches wide, then you divide the required pixels by 5 inches. So 9000 / 5 = 1800.
So you need to scan at 1800 samples per inch to give a 9000 pixels wide scan file.

Also be aware that actual neg image area may not be 5 inches, it may only be 12cm x 9cm which is 4.72in by 3.54 which is a 4:3 ratio. Your 30x24 print is 6:5 ratio so you will be using a crop from the full image area which means you will need to scan at higher than 1800spi. Only you know what the crop will be so you will have to work out actual numbers yourself. If you scan at the scanner native resolution, say 4800spi, then you can crop the scan to the correct aspect ratio and downsize to 9000 pixels width. Or you can measure the scan area and scan at the required resolution to give 9000 pixles width.
If it's a drum scan, then ask for 9000x7200 output file which is what you want. The scan operator will work out the spi for you.

[shannaford]

Thanks so much for the helpful, clear answer, that makes sense the way you've described it.
Cheers,
Scott

[percepts]

just noticed I said 30x24 is 6:5 ratio. It is of course 5:4 ratio but you get the idea.